1. Field of the Invention
The present invention relates to water-based pigment inks, and particularly to water-based pigment inks containing self-dispersing carbon black as a pigment. The present invention also relates to an ink-jet recording method comprising applying such an ink to conduct recording, and ink-jet instruments using such an ink.
2. Related Background Art
As a black colorant for printing inks, carbon black, which is excellent in fastness properties such as water fastness and light fastness, has heretofore been widely used. In order to use carbon black as a coloring material for a water-based ink, it is however necessary to stably disperse the carbon black in an aqueous medium. Since the carbon black generally has poor dispersibility, a dispersing agent is added to an aqueous medium so as to stably disperse the carbon black therein for the purpose of obtaining a homogeneous dispersion system. However, even the method of using the dispersing agent cannot achieve satisfactory dispersibility. Therefore, any ink in which the carbon black is dispersed has involved a special problem that its long-term storage stability is poor.
When an ink is used in ink-jet recording on the other hand, it is generally necessary to eject the ink in the form of stable droplets through a minute orifice of an ink-jet recording head. Therefore, the ink is required to prevent the occurrence of solidification and the like of the ink due to its drying in the orifice of the ink-jet recording head. When the above-described ink containing the dispersing agent is used in ink-jet recording, however, clogging at an orifice and failure in ejection of ink droplets may occur in some cases because a resin composing the dispersing agent is not redissolved in the ink after adhesion to the orifice and the like. Besides, the water-based pigment ink containing the dispersing agent is viscous and hence has involved a problem that it meets with resistance in the course leading to the orifice when conducting continuous ejection and high-speed printing, so that its ejection becomes unstable, and difficulty is encountered on smooth recording.
In order to solve the above problem on the other hand, there has been developed self-dispersing carbon black capable of stably dispersing without using any dispersing agent by introducing a water-soluble group in the surface of carbon black as described in Japanese Patent Application Laid-Open Nos. 5-186704 and 8-3498.